Thin-layer chromatography (TLC) has found increasingly wide-spread acceptance as a rapid method for micro- and ultramicro-analysis. This method has received special impetus by the development of high-pressure liquid chromatography (HPLC). Since the results obtained in TLC can frequently be applied directly to the HPLC technique, for example with respect to analysis periods, R.sub.f values, and eluents, TLC has most recently acquired an additional field of application, in addition to actual microanalysis, namely to serve as a so-called pilot technique for HPLC. This means that initially, with the aid of TLC, suitable separating conditions are determined very rapidly for a specific separation problem, and these conditions can then be substantially applied to HPLC.
However, such an application is possible only if the separating characteristics of plate and column are identical. Sorbents have been developed, however, specifically for HPLC, the surfaces of which are modified by organic groups. These surface-modified (e.g. "reversed-phase") sorbents possess entirely different separating properties from those of non-modified materials and make it possible to execute a multitude of separating operations heretofore infeasible, due to the stepped transitions, depending on the type of modification, ranging from the hydrophilic sorption agent to a material becoming increasingly more hydrophobic, especially in connection with silica gel. By the choice of the organic groups utilized for surface modification, a great variety of surface properties can be conventionally attained.
However, on account of the differing separation properties, it is impossible to apply the results obtained in TLC to these separating materials. Thus, with the objective of being able to utilize, on the one hand, TLC as the pilot technique even for sorbents having a chemically modified surface and of opening up, on the other hand, these variegated separating possibilities also for TLC analysis, the task was under taken of providing TLC separating materials which likewise contain sorbents modified by organic groups.
The attainment of this objective, however, met with considerable difficulties, since the reversed-phase sorbents known from HPLC are unsuitable for producing the customary instant TLC preparations. Such preparations, as is known, are TLC plates or films coated in the usual way with thin layers of the separating material, the latter normally containing binders and optionally indicators. Attempts at producing, by means of the conventional, chemically modified sorbents, TLC layers satisfactory with respect to separating characteristics, adhesive strength, and abrasion resistance have, however, met with failure.
The modified materials show very poor adhesion to the substrate material and cannot be processed with the usual binders.
Also, efforts have been made to react carefully dried, silica gel-coated thin-layer plates with alkyl trichlorosilanes in a chamber sealed off from the outside atmosphere and likewise being kept carefully free of moisture (cf. Journal of Chromatography, 124 [1978]: 257-264). During this step, a chemical modification of the silica gel layer was produced. Due to the complex measures which must be employed in this process on account of the notorious sensitivity to hydrolysis displayed by halosilanes, to keep the reagents as well as the reaction chamber absolutely anhydrous, the process is very expensive and also unsuitable for use on a large technical scale.